DB6,
You are not understanding the responses.
No-one is using your email account to send spam.
Someone is using your email address in the "from" field of the emails they are sending out.
And as others have pointed out, it is trivial to accomplish and there is nothing that can be done about it.
You should be able to look inside the email header and see the actual source mail address, so you might try complaining to the registrar of the domain. But I wouldn't waste my time, if I were you.
SD